1. Field of the Invention
This invention relates to a color image forming apparatus, and more particularly to a color image forming apparatus such as a color copying apparatus having a function for duplicating an image in two desired colors.
2. Discussion of the Background
With respect to a color image forming apparatus, such as a color copying apparatus capable of duplicating an image in two desired colors from an image of an original document, a first reference is made to an image reading apparatus disclosed in the official gazette for Laid Japanese Patent Application TOKUKAI HEI 5-37797. This image reading apparatus includes a function for converting image data into binary value data representing an achromatic color and another function for converting image data into binary value data representing a chromatic color. When reading an image of an original document, the image reading apparatus separates the image into plural color elements of red (R), green (G), and blue (B) and generates corresponding image signals of R, G, and B. The image reading apparatus then analyzes whether the image signals of R, G, and B are in a ratio of one to one to one, and, depending upon the result, selects one of the above-mentioned two functions to be implemented.
A second reference is made to an image forming apparatus disclosed in the official gazette for Laid Japanese Patent Application TOKUKAI HEI 6-152995. This image forming apparatus includes a function for converting image data into a black color signal and a red color signal to reproduce an image in a black and a red color. When reading an image of an original document, the image forming apparatus separates the image into plural color elements of cyan (C), magenta (M), and yellow (Y) to generate corresponding image signals of C, M, and Y. The image forming apparatus further generates a black color signal from a least value and the like of C, M, and Y and a red color signal by subtracting the least value and the like of C, M, and Y from the image signals of C, M, and Y.
A third reference is made to a product of a digital color copying apparatus referred to as PRETER 550 manufactured by Ricoh Company, Ltd., in Japan. This digital color copying apparatus includes a function for converting a color of an image of an original document into a different color of a duplicate image and, further, for designating such a color of the image of the original document and such a different color of a duplicate image when such conversion is executed.
However, in this apparatus a user may have difficulty in obtaining an image in superior color quality on the image reading apparatus of the above-mentioned first reference. This is because analysis as to whether the image signals of R, G, and B are in a ratio of one to one to one is too simple to determine a case of an achromatic color image and another case of a chromatic color image. More specifically, when an image is made of a black color or a high density gray color, each color element included in the image signals of R, G, and B is relatively small and the analysis therefore becomes inaccurate. In this case, an image which should be determined as an achromatic color image is improperly determined as a chromatic color image.
Also, in the image forming apparatus of the above-mentioned second reference, a problem is brought to a user in a way that an image portion to be printed in a red color is printed in a black color. This is because a black signal is generated from a least value and the like of C, M, and Y and a red color signal is generated by subtracting the least value and the like of C, M, and Y from the image signals of C, M, and Y. More specifically, when an image is made of a dark color, such as, for example, a dark red, a dark blue, or the like, each color element of C, M, and Y is relatively small and the analysis of the least value of C, M, and Y becomes inaccurate. In this case, an image which should be determined as a red color image is improperly determined as a black color image.
Also, in the digital color copying apparatus of the third reference, the two color copying function is complex to use. This is because that, in addition to selection of a color desired to be printed in a duplicate image, the user is required to select a color of an image of an original document.
Therefore, a background color image forming apparatus such as a color copying apparatus presently is not capable of reproducing an image in two desired colors in superior quality from an image of an original document, in a relatively simple operation.